I don't have an answer to your question, but if you're just trying to answer up to the buyer, yiu can tell him that you'll be paying sales tax even though you're not charging him.
In other words - you're taking the hit as opposed to passing the charge on to him.
Just make sure he doesn't work for the IRS
+1 That's a good way to shut him up.
Alternatively, you could make up some code provision in the NYS revenue code which will say that your sale isn't covered.
And in fact, there is no way he is supposed to pay sales tax on that. If I sell you my lawnmower, there is no sales tax. The reason is not that it is impossible to track; the reason is that casual sales don't have sales tax. That is why only registered sellers have to pay sales tax, because they are running a real business of it.